Method and means for increasing the power output of a thermionic tube



May 22, 1934. F. B. MacLAREN 1,959,540

METHOD AND MEANS FOR INCREASING THE POWER OUTPUT OF A THERMIONIC TUBEFiled Sept. 2l, 1929 ATTORNEY Patented May 22, 1934 i UNITED STATESPATENT OFFICE METHOD AND MEANS FOR INCREASING THE POWER OUTPUT 0F ATHERMIONIC TUBE Fred. B. 'MacLaren, Malba, N. Y., assignor, by mesneassignments, to Revelation \Patents Holding Company, a corporation ofDelaware *Application september 21, 1929, serial No. 394,172

15 claims. (ci. 179-171) number of turns of ne wire and forms a highresistance which is interposed in the grid cathode or input circuit.Whenever any current flows in the grid cathode circuit, therefore, thereis a potential drop across the secondary of this transformer. When thegrid' of the thermionic tube is negative no current fiows through thegrid cathode circuit but when the grid of the thermionic tube ispositive it will receife some of the electrons emitted from the cathodeand a current will therefore ow. When no current fiows in the gridcathode circuit variations of potential on the grid will be in exactproportion .to the variation impressed on the secondary of thetransformer but when current flows in the grid cathode circuit thevariations of potential on the grid will not be exactly the same as thevariations impressed on the circuit due to the resistance of thesecondary of the transformer which causes the potential drop between thegrid and cathode. In other words since the grid is maintained normallyat zero potential the incoming signal wave will cause it to swing firstnegative and then positive. The grid will respond accurately to thenegative side of the wave and produce true amplification in the platecircuit of the tube but the positive side of the wave will cause gridcurrent to flow between the grid and the cathode and the potential dropin the secondary of the transformer will prevent the grid potentialvarying in accordance with the incoming signal. This results in adistortion of the output current, half the wave coming throughaccurately andthe other half being distorted so that the signalsproduced are muddy and indistinct even though the volume is sufiicientfor perfect hearing under proper conditions.

.To overcome this defect in the amplifier vit has been expedient toplace a negative bias on the grid of the thermionic tube and this biasis made suiciently l large toA maintain the grid always negative andprevent the positive waves of the incoming signal from swinging the gridbeyond the zero mark. The use of the grid bias entirely eliminates thedistortion just described but it has one very serious defect. It limitsthe power output of the tube in such a Way that it not only prevents thetube from producing as great an output as it is capable of doing but italso causes distortion when a modulation of extremely large limits isimpressed on the input circuit as when a The secondary of the inputveryloud or high note is produced at the microphone or other translatingdevice forming the input or source of signals.V This will be readilyseen hereinafter -as the invention is described more in' detail. s

One of the objects of the present invention is to provide a means forincreasing the range of Ypower output of a thermionic tube.

Another object of the invention is to providea method and a means forcompensating for the ordinary distortion produced when the grid of anamplifying tube ispositive so that the grid may swing either negative orpositive and still produce true amplification at the output of the tube.v

Another object of the invention is to provide an input circuit for anaudio frequency thermionic tube which will not have a potential droptherein to cause distortion.

Other objects of the invention and objects relating to methods ofconnecting the various parts of the amplifier together will be apparentas the description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanyingdrawing in which:

Fig. 1 is a circuit diagram illustrating the invention; andv sov Fig. 2is a graphic representation of the plate I current characteristics of athermionic amplier tube.

In Fig. 1 is shown an input transformer 10 having a primary winding 11connected to a source of input cmrent and a secondary winding 12, and anoutput transformer 13 having a primary winding 14 and a secondarywinding 15, these transformers being the ordinary transformers used inan ordinary amplification systern. The primary 14 of the outputtransformer is connected at one end directly to a plate 16 of athermionic amplifying tube 17 while the other end of the primary 14 isconnected to the positive terminal of a B battery 18 whose negativeterminal is connected to a lead 19 which is connected in turn to thecathode 20 of the tube 17, these connections following the usualconnectlons for an amplifier tube. A

Now in order to permit the grid 21 of the vacuum tube 17 to swingpositively Without causing distortion in the output circuit I providewhat I choose to call a compensating tube 22 in the input circuit. Thiscompensating tube 22 may be exactly lI ke the amplifying tube 17 andhaving a grid 23 which I connect to one endA of the secondary 12 of theinput transformer 10 while the other end of this secondary is connectedto the lead 19 and therefore to the cathode 20 of the amplifying tube 17and also to the negative side of the B battery 18. y

The cathode 24 of this compensating tube I connect directly to the grid21 of the amplifying tube 17 by means of the lead 241, while the plate25 is connected to a point on the B battery having a positive potential.26 may be connected across the terminals of the' battery 18 and 'asecond by-pass condenser 27 may be connected if desired between thenegative terminal of the battery and the connection with the plate 25 ofthe tube 22. These condensers provide the feed-back for the plates ofthe two tubes in a manner well known to those skilled in the art.

'Ihe theory of operation of the circuit just described is believed by meto be somewhat as follows: Signals impressed upon the input transformer10 cause a variation of potential on the grid 23 of the compensator tube22. This variation of the grid of the compensator tube causes avariation in the electron stream between the cathode 24 of this tube andthe plate 25 which therefore, as the cathode 24 is connected directly tothe grid 21 of the amplifying tube 17, causes a variation of potentialon the grid 21 of this tube which of course controls the electron streambetween the cathode 20 and plate 16 and therefore the Aoutput of thecircuit. Any variations impressedon the grid 23 of the compensator tube22 therefore will be amplified by the tube 17 and will cause increasedoutput current to flow in the outputcircuit.

Now if .the grid 21 of the amplifier tube 17 is made negative by theincoming signal no electrons will ow between the cathode 20 andgrid 21and no current will flow in the lead 24,. Variations of the incomingvoltage therefore which maintain the grid 2l negative will be trulyamplified by the amplifying tube 17 asis well known in the art. Howeverwhen the grid 21 becomes positive due to the variations impressed uponthe grid,23 by the input circuit, electrons will 'ow between the cathode20 and grid 21 thereby causing a current to flow inthe lead 24';

, However electrons are flowing between the cathode 24 and the plate 25of the compensating tube 22 due to the difference in potentialtherebetween and as the current flowing therebetween is in the samedirection as the current flowing in the wire 24' this current flowsthrough the plate 25 down to the battery 18, to the lead 19,

` and back to the cathode 20 forming a complete path for the gridcurrent. Inasmuch as the resistance between the cathode and plate of thecompensating tube is affected by the variations of the grid of thistube, any drop in potential across this resistance will also vary withthe grid and will therefore cause no distortion when impressed on thegrid of the amplifying tube and hence the amplification of the tube 17is just as accurate for this side of the wave as it is for the negativeside. As a matter of fact I have found that the grid of the amplifyingtube is rarely negative due presumably to the potential differencebetween the cathode and plate of the com pensator tube and theconsequent tendency for current -to flow therebetween.

While this may not be an exact explanation of the theory of operation ofthe invention, the results which are produced indicate to me that itoperates in this manner. These results may be more readily seen byreference to Fig. 2 of the drawing. This figure shows a plate currentcharacteristic curve for a vacuum tube with grid voltage plotted on thehorizontal line and plate current in milliamperes plotted on thevertical line. Before my invention it has been the custom to selectapoint A on the characteristic curve where A by-pass condenser.

the grid voltage is negative to prevent the positive side of theincomingsignal wave bringing the grid beyond the zero grid voltage pointand is between B and C as indicated on the curve.

Any swing lof the grid beyond the point C as would be caused by a largevariation coming over the input circuit will ofcourse cause a distortionin the output circuit.

The characteristic curve however continues in a straight line beyond thezero grid voltage point so that the capabilities of -the vacuum tube areconsiderably greater as to its power output than has been heretoforeused as indicated by the limits B and C. "With my invention the midpointof the grid potential swing, instead of beinglat A. may be at the zeropoint or on the positive side as at A' where it appears to come due tothe operation of the compensating tube and the swing i of the grid isnow not limited to the negative side of the curve but it is free'toswing on both sides making undistorted amplification possible when thegrid swings between the points B on the negative and C' on the positiveside of the curve.

From an inspection of this curve it is immediately evident that I havegreatly increased the power output of a thermionic tube. It will also beevident that any potential drop occurring in the grid circuit of theamplifying tube when the grid is positive varies with the inputvariation so that there will be no distortion in the output circuit ofthe tube.

While I have indicated the positive potential on the plate of thecompensating tube as being less than the potential on the plate of theamplifying tube I have found that this potential may be increased evenabove the amplifying tube potenial with an accompanying increase ofpower output from the amplifying tube.

In the drawing I have not indicated a means of energizing the cathode of.the two tubes used and I prefer to use independently heated cathodetubes commonly called heater type tubes which may be energized byalternating current. These tubes however, should have no connectionbetween the cathode and filament in which cas'e they may both beoperated from the same transformer winding but Where a connection ismade inside of the tube between the cathode and filament there wouldhave to be a separate transformer winding for each tube. Direct currenttubes of course, may be used in which case a separate battery wouldnecessarily have to be used for the compensating tube 22.

I vhave illustrated and described merely one stage of amplification butit is evident that other stages may be added to the construction asshown and described and in fact it is my purpose to put several suchstages in series to provide sufficient output at the loud speaker orother translating device.

Modifications of the invention may be resorted to without departing fromthe spirit thereof and I do not therefore desire to limit myself to whathas been shown and described except as such limitations occur in theappended claims:

What I claim is:

l. In an audio frequency amplifying system a thermionic vacuum tube, aninput circuit for said tube, a transformer secondary coil included insaid input circuit, and means to by-pass any current owing in saidcircuit around said transformer coil, the last mentioned means includinganother thermionic vacuum tube having its cathode conductively connectedto the grid of the first mentioned tube, and circuit arrangements forimparting a positive bias to the grid of the first tube and a negativebias to the grid of the second tube.

2. In an audio frequency amplifying system a thermionic amplifying tube,a grid in said tube, an input circuit connected to said grid, and meansconnected to said input circuit to prevent a distortion oftheamplification of said tubewhen said grid swings to a positivepotential, the last mentioned means including a compensating tube havingits anode directly connected to a source of positive potential and itscathode directly connected to the grid of the said amplifying tube, andcircuit connections including said source of potential tending to biasthe grid of said amplifying tube positively and the grid of said.

an output transformer, a primary winding on said' output transformer, aconnection between one end of said primary winding and the plate of saidsecond thermionic tube, a connectionbetween the other end of saidprimary winding and a source of positive potential, and a connectionbetween the plate of said first thermionic tube and a source of positivepotential.

4. In an amplifying system the combination of a first electron emittingcathode, a control grid associated with said cathode, a second electronemitting cathode, a controlgrid associated with said second cathode, asteady potential source tending to bias the' second grid positive withrespect to its cathode, and circuit connections for causing saidpotential source to maintain the first grid negative with respect to itscathode.

5. A system according to claim 4, in which each cathode is provided withan associated anode, and said potential source serves as the operatingpotential for said anodes.

6. An amplifying system comprising a first electron emitting cathode anda control grid therefor; a second electron emitting cathode and acontrol grid therefor; an anode for the first cathode: an anode for thesecond cathode; a connection between the first cathode and the secondgrid; a series circuit including in sequence the positive terminal of asource of potential, the first mentioned anode, the first mentionedcathode, the second mentioned grid, the second mentioned cathode, thenegative pole of said source; and a connection from the said negativepole to the first grid whereby said first grid is negatively biased withrespect to the first cathode.

'1. An amplifying system comprising a first cathode, a first grid, asecond cathode, a second grid, means for connecting the respective gridcathode spaces in series, said means including an input impedance, andcircuit connections and a source of steady potential for simultaneouslyimparting some positive bias to the second grid and some negative biasto the first grid, substantially independently of the signal variations.

8. An amplifying system comprising a rst electron emitting cathode and acontrol grid as'- sociated therewith, a second electron emitting.cathode and a control grid associated therewith,

the first mentioned grid being somewhat negatively biased with respectto its cathode substan- 10. A system according to claim 8 in which thefirst cathode and second grid are conductively connected together by aconnection of substantially z'ero impedance.

11..In a system of the character described a thermionic amplifyingtriode, an input circuit for said triode comprising a compensatingtriode, a connection from the cathode of said amplifying triode to thenegative terminal of the anode supply, a connection of substantiallyzero impedance from the grid of said amplifying triode to the cathode ofsaid compensating triode, a connection of substantially zero impedancefrom the anode of said compensating triode to a positive terminal of theanode supply, whereby said anode supply tends to bias the grid of thecompensating triode negatively, and the grid of the amplifying triodepositively.

12. A system of the character described comprising a first triode, asecond triode, the `anodecathode space of the second. triode beingconnected across the grid-cathode of the first triode to form part ofthe input impedance of the said first triode, and circuit connectionstending to bias the grid of the first triode positively and the grid ofthe second triode negatively.

13. A system according to claim 12, in which the means for biasing onegrid positively and the other grid negatively includes the source ofanode potential for both triodes.

' 14. A system of the character described comprising an amplifyingtriode, a compensating triode, a source of anode potential for bothtriodes, a connection of substantially zero impedance from the anode ofthe compensating triode to a positive terminal of said potential source,a connection of substantially zero impedance from the cathode of thecompensating triode to the grid of the amplifying triode, means forimpressing the signals to be amplified on the grid of the compensatingtriode, and circuit connections for causing the signal Variations on thegrid of said compensating triode to be in substantially the same phaseas the signal variations on the grid of the amplifying triode.

15. In an audio frequency amplifying system, a thermionic amplifyingtube, a grid in said tube, an input circuit connected to said grid, asecond amplifier tube connected in said input circuit, a grid andacathode in said second tube, said grid and cathode being connected inseries with said first mentioned grid, a source of positive potentialconnected to the plate of said second amplifier tube, and means wherebysaid source of potential tends to bias the grid of the first tubepositively andthe grid of the second tube negatively, the last mentionedmeans including a conductive connection from the grid of the first tubeto the cathode of the second tube, and a connection from the grid of thesecond tube to the cathode of the first tube.

FRED. B. MACLAREN.

Disclaimel Y 1,959,540-lred B. M aeLawm, Malba, N. Y. METHOD AND MEANSFon INCREAS- ING THE Powxpn OUTPUT oF A THERMIONIC TUBE. Patent datedMay22, 1934. Disclaimer filed Nov. 2, 1950, by the assignee, RemoElectro/nie, Ine.

Hereb` enters this disclaimer to claim 14 of said patnt.

[ #cz'al Gazette December 5, 1.950.]

